The present invention is related generally to circuits for protecting electrical circuits, elements or devices from surges in power lines and particularly to a circuit for detecting and absorbing electrical power surges in relatively high power applications.
It is well known that electrical circuits often experience surges in the power supplied to the circuit. These power surges may be due to a wide variety of disturbances, such as equipment failure or inadvertent destruction, or natural disturbances such as lightening strikes. Such surges often occur quickly, leaving little time to respond in a fashion to protect electrical equipment connected to the surging line.
To protect electrical equipment from surges, it is known to use varistors, which are devices in which electrical resistance decreases in response to increasing current. The varistors used in surge circuits may be placed in a circuit such that when an increase in the power to the circuit is sensed by the varistor, the varistor's electrical resistance decreases and the surging power is shunted across the varistor and away from more surge sensitive components.
The maximum voltage that can be continuously impressed across a varistor is generally limited by its steady state power dissipation rating. Under surge conditions, a varistor can absorb a pulse of energy determined by its mass and peak allowable temperature. Typically, the varistor clamping voltage is in the order of twice its steady state rating.
An efficiency which has been experienced in the use of such devices in surge suppression is the loss of power across the varistor when the power is not in a surge condition. Such a loss of power could be reduced if the varistor was actively switched into a circuit to be protected as needed; however, because varistors typically conduct surge currents of hundreds to thousands of amps, it has generally not been practical to combine varistors with active switching devices.
Another difficulty which has been experienced in the prior art is the use of varistors in surge suppression circuits having high energy repetitive pulses. Because such pulses often produce a gradual increase in the leakage current and nominal clamping voltage, high energy repetitive pulses in varistor circuits can lead to thermal runaway and catastrophic failure.
It is accordingly an object of the present invention to provide a novel circuit and method for obviating these and other difficulties in the prior art.
It is another object of the present invention to provide a novel circuit and method for suppressing surge voltages which are a relatively small increase over the steady state applied power.
It is a further object of the present invention to provide a novel circuit and method for rapidly switching a varistor into a circuit upon the detection of a rising surge voltage.
It is still another object of the present invention to provide a novel circuit and method for suppressing transient voltages which are repetitively generated as a consequence of normal circuit operation.